I can't remember where I saw this, but sometime today, I saw something like: "The Sharingan helps the Uchiha counter genjutsu" and i've been thinking about it for a while now. Can anyone explain how the Sharingan helps the Uchiha counter genjutsu?
I understand that users of the Sharingan can easily use genjutsu on others, but how does the Sharingan help them counter the effects of genjutsu?
The Sharingan sees chakra as color, and Genjutsu works off chakra, so it stands to reason that the built in Genjutsu detection is actually chakra detection. If the Genjutsu caster didn't personally add it, an illusion of a person would to a Sharingan look like a hollow puppet. It would be lacking the chakra network all living people must posses to live (recall chakra exhaustion results in death, which is shown twice during Pain's invasion), thus obviously be Genjutsu.
Though, as Islam Elshobokshy says, it does not render you immune to Genjutsu. It only makes it easier to Detect Genjutsu. As Sasuke says in one of the dubs, "My eyes can see through Genjutsu". In Sasuke vs Itachi, we see both of them play along to each others Genjutsu, but also clearly show they knew it was all an illusion in the beginning of the fight. Zetsu commented on how they just stood around not moving. The Tsukuyomi however seemed to fool Sasuke, until he broke through it.
Black Zetsu, during the fight between Itachi and Sasuke, states that the sharingan is just like any weapon that a shinobi possesses.
It's power depends upon how powerful it's weilder is.
So, yeah it's definitely possible for a Sharingan user to succumb to a genjutsu casted by another Sharingan user. A Sharingan genjutsu casted on someone having a sharingan can be easily broken by the victim, if he is powerful enough to counter it.
If not, then he'll definitely succumb to it.
There are numerous examples :
The most common ones coming to my mind involve Itachi's Tsukoyomi, which granted is a Mangekyo Sharingan ability, but it's ultimately a genjutsu.
Itachi used the Tsukuyomi several times on Sasuke, and on Kakashi as well. It's also an exceptional genjutsu though, which can only be countered by a blood related Sharingan user. Even so, Sasuke is not able to counter it, during his fight with Itachi (itachi let him out of it, after assessing his potential).
Reanimated Itachi and Sasuke cast on each other their respective Genjutsu, to break out of Kabuto's genjutsu.
Then there's this ultimate genjutsu, The Kotoamatsukami. Ability of Shisui Uchiha's Mangekyou Sharingan, reanimated Itachi has this cast upon himself during the war. This genjutsu is unbreakable by any means, at least as stated in the manga.
